This invention relates to fluidized bed reactors, and more particularly, to a method to reduce the emission of halogen compounds in gaseous products resulting from the combustion of halogen containing fuels in fluidized bed reactors.
Substantial efforts have been made to reduce emission of halogen compounds An gaseous products resulting from the combustion of halogen containing fuels, such as certain coals, industrial and municipal wastes, to comply with environmental regulations. In general, there are three prior art methods to reduce halogen emissions in flue gases: wet scrubbing, spray drying and dry-solids contact. In both the wet scrubbing and spray drying processes, a reaction vessel provides a region in which an interaction between a mixture of water and an alkaline sorbent-material, such as lime, and the flue gases can take place. The mixture of water and sorbent material forms an alkaline solution which is highly conducive to the absorption of halogen compounds, such as hydrogen halide. Unfortunately, both the wet scrubbing and spray drying processes suffer from major problems with scaling and corrosion resulting from the presence of an aqueous solution phase. The dry-solids contact process, while avoiding the problems associated with the aqueous solution phase, suffers from a relatively low halogen removal efficiency due to relatively slow solid-gas reaction kinetics.
The dry-solids contact process typically involves the injection of a dry, alkaline sorbent-material, such as limestone, into the combustion vessel of a fluidized bed reactor. Unfortunately, only the most reactive halogen, fluorine, is retained in the sorbent material while only a small portion of the most abundant halogen, chlorine is retained due to the elevated temperatures disposed within the combustion vessel.
In other known dry-solids contact processes, a dry, alkaline, sorbent material, such as lime, is introduced into the flue gases upstream from a baghouse and the sorbent material is distributed over the input side of a baghouse filter. The filter thus provides a region in which interaction between the sorbent material and the flue gases can take place.
This latter process of dry scrubbing is generally considered too expensive for use in many industrial fluidized bed reactors because it incurs a significant cost disadvantage by using lime instead of limestone since the cost of lime is as much a ten times the cost of the limestone.
Accordingly, there remains a need in the art for a dry-solids contact process to remove halogen compounds from flue gases without incurring the additional cost of using lime.